In general, in conventional seamless tube rolling mills, the rolling stands are structurally independent one from another and can be individually moved off the mill in order to allow their replacement. In a preferred embodiment, the rollers of such stands have coplanar axes of rotation which lie on a plane orthogonal to the rolling axis; such a rolling mill is commonly referred to as a continuous rolling mill.
In general, in the seamless tube making industry, it is recognized that proper performance of the rolling process is closely dependent on the action being applied by the roller grooves at each rolling stand.
More particularly, it is recognized that the geometric tolerance and the surface finish of a tube depend on the difference between the tube rate of advancement along the rolling axis and the peripheral speeds of the rollers as measured at several locations of the grooves, in contact with the tube.
The commercial production of seamless tubes is currently carried out mainly on mandrel-type continuous rolling mills having a set of successive stands each provided with two driven rollers; such rollers are supported by an external structure, opposite one another and have parallel axes of rotation. In this specific case, the contact of the tube to be processed with the groove of one such roller occurs approximately over one half the external circumference of the tube.
In recent years, on a purely experimental basis and alternatively to the above-mentioned approach, the feasibility of continuous rolling mills provided with rolling stands having more than two rollers was investigated.
In general, in the last-mentioned embodiment of the rolling mill, contact between the profile of the roller grooves and the tube to be processed occurs over an arc of said external circumference whose length is inversely proportional to the number of the rollers in each stand.
Thus, in the particular instance of a three-roller stand, the profiles of the roller grooves will be active over an arc being approximately one third the external circumference of the tube.
The development of rolling mills equipped with stands having more than two rollers is of great interest because it has been verified, both theoretically and experimentally, that the shorter the length of the tube arc being worked upon by a single roller, the better the resultant tube surface finish and thickness tolerances.
This explains the efforts being currently made in the art in order to provide rolling mills which embody this novel technological concept.
It should be considered, however, that while having more than two rollers enhances mill performance, as the number of the rollers in each rolling stand is increased, the technical difficulties encountered in engineering the rolling mill also increase significantly. As an example, the construction of three-roller stands already involves technical difficulties which have yet to be fully overcome; among these difficulties are the problems posed by simultaneously driving three rollers and adjusting their distances from the rolling axis.
In fact, three-roller stand mills tried or known heretofore fail to provide such adjustment feature to an adequate degree to make the rolling mills suitable for industrial applications; that is, the mills are too rigid, and unsuitable for coping with the different operating conditions required by the tubes, or pipes of industrial production.